Natural antibodies play a major role in providing protective host immunity. A significant portion of natural antibodies present in all humans, apes and Old World primates are specific for the carbohydrate antigen Gal11-3Gal21-4GlcNAc-R, or 1Gal. Indeed, 1Gal-specific natural antibodies comprise one to eight percent of circulating immunoglobulin in humans. Based on the universal high-titer presence of these antibodies, we hypothesized that it might be possible to utilize this pre-existing antigen-specific repertoire to augment the immunogenicity of antigens in order to improve the efficacy of vaccines. To test this hypothesis, we used 1Gal-deficient mice (GT0 mice), which like humans produce 1Gal-specific natural antibodies, to analyze whether conjugation of a poorly immunogenic antigen, such as bovine serum albumin (BSA), to 1Gal could affect its immunogenicity in vivo. Immunization of GT0 mice with BSA conjugated to 1Gal (1Gal-BSA) led to a T and B cell response to BSA that was significantly greater than that observed following immunization of control mice without the need for adjuvant. The ability to produce 1Gal-specific antibodies also led to an enhanced cytotoxic T lymphocyte anti-virus response following challenge of mice with murine leukemia virus- transformed cell lines expressing 1Gal. These data suggest that pre-existing 1Gal-specific antibodies encoded for in the natural antibody repertoire can be used to increase B and T cell responses to poorly immunogenic antigens that have been modified to express 1Gal epitopes without the need for adjuvant. The central hypothesis of this proposal is therefore that this pre-existing antibody repertoire can be used to improve vaccine strategies for pathogens. The specific aims are: to determine the mechanism by which 1Gal-specific natural antibodies increase the immunogenicity of antigens modified to express 1Gal; determine if pre-existing 1Gal-specific natural antibodies can be used to improve vaccines for pathogens, and; determine the effectiveness of immunization with 1Gal-modified antigens in immunocompromised hosts.